In an aromatic alkylation process, aromatic hydrocarbons, such as benzene and toluene, react with alkylating agents, such as ethylene and propylene, in the presence of a silica-containing molecular sieve catalyst to produce alkyl-substituted aromatics, such as ethylbenzene and ethyltoluene. Chemical intermediates resulting from aromatic alkylation processes include isopropylbenzene, which is used in the manufacture of phenol, and vinyl toluene monomers, which are used in the production of a variety of styrenic polymer materials. In regard to transportation fuels, the use of alkyl-substituted aromatics as blending agents for gasoline expand product volume and increase octane values. Further, aromatic alkylation processes provide a cost effective manner of reducing the amount of benzene in gasoline.
In the past, Friedel-Crafts type catalysts were used as the alkylation catalyst in aromatic alkylation processes. However, the use of these catalysts have numerous disadvantages, including corrosion problems, high regeneration costs, low yields of alkylates boiling in the gasoline range, and complicated separation processes of alkylated products.
These disadvantages can be avoided by employing processes that use crystalline zeolite catalysts which are non-corrosive, and from which the alkylation products can be more readily separated. Alkylation of aromatic hydrocarbons using a crystalline zeolite catalyst has heretofore been described in U.S. Pat. No. 2,904,607 which refers to alkylation of aromatic hydrocarbons with an olefin in the presence of a crystalline metallic aluminosilicate catalyst having uniform pore openings of 6-15 Angstroms.
While crystalline zeolite catalysts represent a distinct improvement over previously suggested Friedel-Crafts type catalyst, they have the disadvantage of producing unwanted quantities of impurities along with the desired alkyl aromatic product, thereby decreasing the overall Yield and selectivity for the product. Another problem with these types of catalysts is that they are subject to rapid deactivation, particularly under vapor phase reaction conditions where gaseous olefins can compete with aromatics for active catalyst sites and result in coking of the zeolite. Consequently, many prefer liquid phase alkylation conditions, for example, U.S. Pat. Nos. 3,641,177, 3,251,897, and 3,631,120.
U.S. Pat. No. 4,849,569 applies reactive-distillation to aromatic alkylation. Since in reactive-distillation the reaction is occurring concurrently with separation, the initial alkylation product is removed as soon as it is formed. Consequently, decomposition of the alkylation product and oligomerization of the olefins are minimized. Another advantage of the application of reactive-distillation to aromatic alkylation is increased energy efficiency due to the exothermic heat generated by the alkylation reaction being used to assist in separation.
Initial reactive-distillation processes did not have concurrent reaction and separation. In U.S. Pat. No. 3,579,309 there is disclosed a distillation column for carrying out organic chemical reactions using a catalyst, the column being formed with catalyst-receiving reaction vessels which are arranged outside the column between individual column tray outlet and inlet openings. Since the reaction and separation steps are not concurrent, this process is not considered energy efficient.
In U.S. Pat. Nos. 3,629,478, 3,634,534, and 3,634,535, there is disclosed a process that contacts reactants with a heterogeneous catalyst in the downcomers of the reactor. While this arrangement permits the reaction and separation to be performed in the same vessel, the practical design of downcomers to convey liquid through the catalyst with the limited liquid head available can result in very inefficient use of the space within the distillation reactor.
In U.S. Pat. No. 3,506,408, a multistage reaction apparatus is shown. The apparatus comprises a liquid feed inlet at the top of the apparatus, a gas inlet at the bottom of the apparatus, and a plurality of perforated trays containing catalyst beds positioned along the length of the reactor. The liquid passes downward through the catalyst on the trays and the gas zig-zags around the trays such that there is essentially no countercurrent contact of liquid and gas within the catalyst beds. As a result there can be very inefficient fractionation of vapor and liquid components.
All of the aromatic alkylation processes discussed hereinabove also have the additional disadvantage of having to take the reactor off line in order to replace deactivated catalyst. This can be a difficult and expensive process for fixed-bed catalyst systems, particularly the catalyst system disclosed in U.S. Pat. No. 4,849,569. In that system, particulate catalyst is contained in an array of closed cloth pockets supported by wire mesh. A typical column can have hundreds of these arrays that will need to be replaced individually by hand.
There is a need for an aromatic alkylation process that does not require shutting down the reactor to replace deactivated catalyst.